1. Field of the Invention
The present invention relates to the safety structure of a centrifugal separator and, more particularly, to a grounding structure for enhancing the electrical safety of a centrifugal separator.
2. Description of the Related Art
The centrifugal separator is configured so that a rotor caused through a tube and a bottle to hold a sample to be separated is accommodated in a rotor chamber (rotating chamber), and that the rotor is rotated at high speed by a drive unit, such as a motor, in a case where an opening portion of the rotor chamber is hermetically closed by a door, to thereby separate and purify the sample held by the rotor. The rotational speed of the rotor varies with the use thereof. Generally, there are provided families of products having rotational speeds that widely range from a relatively low speed, the maximum value of which is about several thousands revolutions per minute (rpm), to a high speed, the maximum value of which is about 150,000 rpm.
FIG. 5 shows the configuration of a known centrifugal separator. The centrifugal separator comprises a motor housing (casing) 5 of a motor 5a serving as a rotary drive source, a rotating spindle (shaft) 5b rotatably connected to the motor 5a, a rotor 1 fixed to the rotating shaft 5b and adapted to hold a sample to be separated, a rotor chamber 2 adapted to accommodate the rotor 1 and to have an opening portion 2a in the top surface thereof, a door 3 openably/closeably provided in the opening portion 2a of the rotor chamber 2, a door lock mechanism 4 adapted to restrict the opening/closing of the door 3, a door opening/closing detector 12 adapted to detect the opening/closing of the door 3, and a control unit 60 adapted to control the motor 5a and the door lock mechanism 4. The motor housing 5, the rotor chamber 2, the door lock mechanism 4, and the control unit 60 are accommodated in the casing (frame) 10.
When the door 3 of the rotor chamber 2 is opened, a user may touch the rotating shaft 5b of the motor 5a and the rotor 1 that may electrically be conducted to the rotating shaft 5b. Thus, generally, an electrically insulating layer is provided between the winding of the motor 5a and the rotating shaft 5b to thereby prevent a user from getting an electrical shock.
Further, in consideration of the possibility that such an insulating layer may cause dielectric breakdown to thereby generate a power supply voltage on the rotating shaft 5b, electric shock guard means are doubled by electrically grounding the housing 5 of the motor 5a through a ground connection wire 8. Usually, the casing 10 of the centrifugal separator is connected to a ground connection wire 9. Thus, the ground connection wire 8 of the motor housing 5 is electrically connected to a part of the casing 10 of the centrifugal separator placed in the vicinity of the motor housing 5. Incidentally, the value of a leakage current of the centrifugal separator is limited to a value, which is predetermined according to JIS (Japanese Industrial Standards) safety standard or to IEC (International Electrotechnical Commission) safety standard, or less. Also, it is required to place a plurality of electric shock guard means at members of a centrifugal separator, which have possibilities of being touched by users.
Meanwhile, the motor 5a generates a leakage current due to the stray capacitance generated between the wiring and the housing 5 of the motor is generated in the ground connection wire and has the property of increasing the entire leakage current flowing in the casing of the centrifugal separator. Thus, it is sometimes difficult to limit the value of the leakage current to a regulation value, which is predetermined according to the aforementioned safety standards, or less. Consequently, the leakage current is reduced by using an insulation transformer 13 that needs no ground connection wire at a secondary side thereof, as shown in FIG. 4.
Furthermore, although it is effective in reducing a leakage current to remove the ground connection wire 8 of the motor housing 5, it is necessary for realizing the plurality of electric shock guard means after the removal of the ground connection wire 8 of the motor housing 5 to provide double insulation or reinforced insulation, which is equated with the double insulation, in the motor itself.
Incidentally, regarding the related art, JP-UM-B-60-20753 discloses the technique of preventing occurrence of an electric shock by providing an electrical insulating layer between the rotor winding and the rotating shaft of a motor, or what is called a double insulation technique of constructing also a motor casing by an insulating material. Further, JP-A-9-187428 discloses the technique of preventing the generation of a leakage current by using the insulation transformer.
The aforementioned structure of the motor, which employs the double insulation or the reinforced insulation, is advantageous in removing the ground connection wire of the housing of the motor and in reducing the leakage current and has a problem in that the cost thereof increases because of the structure thereof.
Further, even when the double insulation or the reinforced insulation of the motor or the motor housing is performed, the motor itself is sometimes charged. Electrification voltages therefor are regulated according to the JIS standard and the IEC standard. In a case where the electrification voltage exceeds a predetermined regulation value, there is a problem that the ground connection wire of the motor housing cannot be removed to prevent the electrification of the motor. Conversely, in a case where the value of a leakage current exceeds a predetermined regulation value when the ground connection wire of the motor is used, there is another problem in that it is necessary to take countermeasures against the leakage current, for example, to distance a drive circuit, which generates a leakage current, from the casing of the centrifugal separator, or to prevent the generation of a leakage current by readjusting the constant and the configuration of an electric noise suppression circuit that generates a leakage current.